Plasma is an ionized gas. Active species of ions, electrons, high-energy neutrals, radicals as well as ultra-violet emission in plasmas can be used for modification of material surfaces such as surface activation/inactivation, adhesion improvement, wettability enhancement, printability improvement, surface cleaning, hardening, cross-linking, curing, polymer-chain secession, coloration, roughening, ashing, etching, sterilization, thin film deposition, material synthesis (particle formation at the surface etc.) etc.
Plasma surface modification can usually be divided into two categories with opposite effects, depending mainly on the process gas(es) used. The first one mainly ablates the surfaces, and is usually called “plasma treatment”, “plasma surface modification”, “plasma ablation” (R Li et al. Composites Pt.A 28A (1997) 73-86), or “non-polymer-forming plasma” (N Dilsiz J Adhesion Sci. Technol. 14(7) (2000) 975-987). The second one is usually called “plasma polymerization”, “polymer-forming plasma” (N Dilsiz J Adhesion Sci. Technol. 14(7) (2000) 975-987) or plasma enhanced chemical vapour deposition (PECVD). In the following “plasma surface modification” is meant to cover both types while “plasma treatment” is used for the first one and “plasma polymerization” is used for the second one.
If the used gas(es) has high proportions of carbon and hydrogen atoms, double- or triple-bonds in its composition such as methane, ethylene, acetylene and ethanol, or if they are precursors such as metal-organic (organometallic) gas(es), the plasma often results in plasma polymerization or PECVD. Here, metal-organic gases are those which contain a metal, particularly compounds in which the metal atom has a direct bond with a carbon atom. Otherwise, the plasma will have a tendency of ablation (plasma treatment).
A variety of plasmas exists, including direct current plasmas, capacitively coupled plasmas, pulsed plasmas, magnetron plasmas, electron cyclotron resonance plasmas, inductively coupled plasmas, helicon plasmas, helical resonator plasmas, microwave plasmas, and plasma jets (see e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658.). Many of them are operated at low pressures, suffering from the drawbacks that they require expensive vacuum systems. Furthermore, methods are only well-developed for batch or semi-batch treatments. To overcome these drawbacks an atmospheric pressure plasma surface modification system can be used that not only avoids the need for vacuum equipment but also permits both the surface modification of large objects and production line continuous surface modification (see e.g. C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30.).
A prior art plasma application system is shown in FIG. 1 and is explained in more detail in the following. FIG. 1 illustrates an example of capacitively coupled plasma of the well-known so-called dielectric barrier discharge (DBD) type usable at atmospheric pressure.
Other types or variations of plasma sources include dielectric barrier discharges (DBDs) with a single dielectric barrier located substantially in the middle between the two electrodes or with a single dielectric barrier covering only one of the electrodes. Such plasma sources are typically also referred to as volume discharge (VD) sources where a micro-discharge can take place in thin channels generally randomly distributed over the electrode- and/or dielectric-surface. Other DBD plasma sources include so-called surface discharge (SD) plasma sources typically comprising a number of surface electrodes on a dielectric layer and a counter-electrode on the reverse side of the dielectric layer. Such SD plasma sources may include a so-called SPCP (Surface-discharge-induced Plasma Chemical Processing) discharge element or CDSD (Coplanar Diffuse Surface Discharge) element. In a SPCP, electrodes are attached on the dielectric(s) and in a CDSD the electrodes are embedded in the dielectric(s).
Other types of plasma sources are e.g. so-called plasma torches such as arc plasma torches, cold plasma torches (see e.g. H Mortensen et al. Jpn. J Appl. Phys. 45(10B) (2006) 8506-8511.), atmospheric pressure plasma jet (APPJ), pencil like torches, barrier torches, and microwave torches (see e.g. C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30.). Yet another type of plasma source is the so-called gliding arc (see for example A Fridman et al. J. Phys. D Appl. Phys. 38 (2005) R1-R24).
Additional types of plasma sources are low pressure plasmas, corona discharge (see e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658) and microplasmas (see e.g. V Karanassios Specrochimica Acta Pt.B 59 (2003) 909-928). See e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658, U Kogelschatz Plasma Chem. Plasma Proc. 23(1) (2003) 1-46, C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30 and A Fridman et al. J. Phys. D Appl. Phys. 38 (2005) R1-R24) for further details of plasmas and atmospheric pressure plasmas.
The two articles ‘Ozone generation by hollow-needle to plate electrical in an ultrasound field’, J. Phys. D: Appl. Phys. 37 (2004) 1214-1220 and ‘Ultrasound and airflow induced thermal instability suppression of DC corona discharge: an experimental study’, Plasma Sources Sci. Technol. 15 (2006) 52-58 by Stanislav Pekárek and Rudolf Bálek disclose suppression of DC corona discharge where ultrasound or ultrasound combined with an airflow is used in connection with a hollow needle-to-plate electrode system to activate the corona discharge for ozone production. The article ‘Improvement of Charging Performance of Corona Charger in Electrophotography by Irradiating Ultrasonic Wave to Surrounding Region of Corona Electrode’ (Kwang-Seok Choi, Satoshi Nakamura and Yuji Murata Jpn. J. Appl. Phys. 44(5A) (2005) 3248-3252.) discloses improvement of the charging speed of a corona charger in electrophotography using an ultrasonic wave where the ultrasonic wave increases the charge density on an insulator layer of a coated aluminum drum used instead of a photoreceptor drum used for printing. At least some of the findings in the articles have also been disclosed in patent application CZ 295687.
The ultrasonic generators disclosed in three above-mentioned articles are based on piezoelectric transducers. No mention is given of specific or preferred sound pressure levels of the emitted acoustic waves or ultrasonic waves or the advantages thereof.
Furthermore, the two first articles mention that the acoustic pressures developed by ultrasonic layouts are, respectively, of the order of 2 and 10 kPa near the emitting surface of the transducer at the frequency of generated ultrasound of 20.3 kHz. In the third article, the ultrasonic generator is a 28-kHz 50-mm-diam and 80-mm-height bolt-clamped Langevin-type piezoelectric transducer. The maximum input power is 50 W. These values give an estimation of the emitted acoustic pressure value to be approximately 2 kPa. The pressure values of 2 and 10 kPa correspond to very high sound pressure levels of 160 and 174 dB above the reference pressure of 2×10−5 Pa. It can be estimated that the above-specified acoustic pressures at the above frequency correspond to ultrasound intensities of 4.4 and 20 kW/m2 or the sound intensity levels of 156 and 163 dB above the reference intensity of 1 pW/m2.
This characterizes the ultrasonic acoustic waves being applied in these three articles as high-intensity.
However, the acoustic power provided according to all three articles is in fact too low and too localized to allow for uniform minimization and/or elimination of a laminar boundary layer over a relatively substantial area of an object to have its surface modified by plasma.
The similar situation can be outlined regarding the patent specification U.S. Pat. No. 6,391,118. It discloses a method for removing particles from the surface of an article in an apparatus using corona discharge. The particles are supplied with an electric charge and subsequently an ultrasonic wave or gas stream is applied onto the surface of the article while an electric field is applied for driving away the electrically charged solid particles from the surface. The application of an ultrasonic wave and/or gas further facilitate the removal of the electrically charged solid particles. The variety of ultrasonic generators (oscillators) here includes a piezoelectric oscillator, a polymer piezoelectric membrane, an electrostrictive oscillator, a Langevin oscillator (that is as mentioned above just a special type of piezoelectric transducers), a magnetostrictive oscillator, an electrodynamic transformer, and a capacitor transformer. Use of such oscillators provides acoustic power that is low (no more than 50 W) and localized. It is too low and localized to allow for uniform minimization and/or elimination of a laminar boundary layer over a relatively substantial area of an object to have its surface modified by plasma. Moreover, no disclosure is given of a specific or preferred sound pressure level of the emitted ultrasonic waves or the advantages thereof. Furthermore, plasma surface modification is not addressed in this specification.
Additionally, no mention or discussion of a laminar boundary layer around the object to be modified is given. The presence of this boundary laminar layer hinders the excess of energetic plasma particles (and hence the mass and energy transfer) to the surface of the object and thereby reduces the efficiency of the plasma treatment. It is important to eliminate or reduce the laminar boundary layer substantially uniformly over a substantial area of the object to be modified by plasma.
Patent application US 2003/0165636 discloses a process for atmospheric pressure plasma surface modification of an object's surface where excitation of the surface to be treated is done so that it vibrates and undulates thereby activating the application of plasma. The energy for excitation of the surface may come from the process of creating the plasma, from an external source, or from a combination thereof. The energy for excitation of the surface may come from a vibration generator brought in contact with the object to be treated or by indirect contact from a vibration generator emitting acoustic waves, e.g. ultrasonic waves, to the object to be treated so that it provokes turbulent plasma. No disclosure is given of a specific or preferred sound pressure level of the acoustic waves or the ultrasonic waves or the advantages thereof. Therefore, exciting surface vibrations and undulations, or in other words, generation of guided and surface acoustic waves on the object is suggested in order to intensify a plasma treatment. Correspondingly, it is disclosed that the vibration of the surface to be treated can be the result of excitation at one or several eigenfrequencies and their harmonics associated with the body of the object to be treated. Thus, either the range of the characteristic dimension of the modified object (primarily its thickness) is strictly limited by the operating frequency of the used source of acoustic energy, or the said frequency is strictly determined by the dimension of the object. It is also disclosed that the vibration of the surface can also result from forced frequencies when an external generator of acoustic waves emits frequencies that are not harmonics of the eigenfrequencies of the object to be treated. This signifies generation of surface acoustic waves (primarily the Rayleigh surface waves).
The following procedures of transfer of acoustic power into ambient gas/plasma are mentioned:                1. External acoustic generator→Treated object surface vibration→Gas molecules (plasma particles) vibration.        2. Generation of the treated surface object vibration directly, for instance through a direct acoustic contact→Gas molecules (plasma particles) vibration.        
Both procedures require acoustic waves to overpass the solid/gas interface at least once. However, due to more than four-orders-of-magnitude difference in acoustic impedance for a solid and a gas, most of generated acoustic power cannot be emitted (and especially re-emitted) into the gas atmosphere and remains in a solid being ultimately converted into thermal energy. Thus, it is not possible in this way to generate sound or ultrasound in the air with a power that would be enough to remove or reduce the laminar boundary layer on sufficiently large surfaces. Moreover, no mention or discussion of a laminar boundary layer near to the surface of the object to be modified is given. The presence of this boundary laminar layer hinders the access of energetic plasma particles (and hence the mass and energy transfer) to the surface of the object and thereby reduces the efficiency of the plasma treatment. Therefore, it is of prime importance not simply to “shake” the surface up and provoke uncontrolled turbulent plasma with unknown efficiency and spatial distribution in such a way, but rather to eliminate or reduce the boundary laminar layer directly, efficiently, and substantially uniformly over a substantial area of the object to be modified by plasma.
Patent specification CN 1560316 discloses a process for ultrasonic plasma-spraying for controlling coating porosity and improving coating bond strength where an ultrasonic excitation source is connected in parallel with an anode and a cathode of the spray gun of the plasma-spraying device and where the sprayed powder is heated and the ultrasound acts on the plasma and simultaneously on the melting sprayed powder. No mention or discussion of a boundary laminar layer over the surface of the object to be modified is given. The presence of this boundary laminar layer hinders the access of energetic plasma particles (and hence the mass and energy transfer) to the surface of the object and thereby reduces the efficiency of the plasma surface modification.
The generated acoustic power is relatively low (at any rate, below 100 W) because the power applied to the acoustic wave transmitter is actually 100 W, and the efficiency of sound generation in a gas atmosphere cannot exceed ˜30% even for the most effective gas-jet ultrasonic transmitters, not to mention other methods. This is too low power to efficiently allow for substantial uniform minimization and/or elimination of the laminar boundary layer over a substantial area of an object to have its surface modified by plasma.
Patent application JP 11335869 A discloses a process for surface treatment and a device therefore where activated species in gas form is generated in a plasma and where the activated species in gas form is subjected to ultrasound and brought in contact with a surface. So a surface treatment by a gas and not surface treatment by a plasma is disclosed.
A principal impediment to the transfer or transmission of energy and/or mass from a plasma to a solid surface is the laminar boundary layer between the plasma and the object to be treated, which adheres to the solid surface. Even when the motion of the gas is fully turbulent, a laminar boundary layer exists (as explained in greater detail in connection with FIGS. 3a and 3b) that obstructs mass transport and/or energy transfer. While various methods and types of apparatus have been suggested for overcoming the problem such as by means of driving the plasma with sonic waves and vibrating the solid object with external vibration generators, these methods while being effective to some extent, are inherently limited in their ability to generate an effective minimization of the laminar boundary layer and at the same time covering an area large enough to make the method efficient.
None of the mentioned prior art disclosures specify an acoustic power level sufficient to efficiently allow for substantial uniform minimization and/or elimination of the laminar boundary layer over a substantial area of an object to have its surface modified efficiently by plasma. Furthermore, the prior art involving piezoelectric transducers or other transducers involving a solid to transfer the energy, only provides the energy in a very localized fashion, e.g. very close to the piezoelectric transducer (or other solid transducer) and is therefore unsuitable for uniform surface modification.